Prime Minister Jacinda Ardern says there is nothing explosive in the secret letter from Julie Anne Genter expressing her views on a multi-billion-dollar transport package for Wellington.

And Ardern is downplaying the letter's importance, saying the National Party is playing politics by making it a bigger deal than it is.

Genter, the Associate Transport Minister and a Green MP, sent the letter to Transport Minister Phil Twyford in March before the local council voted to support its $6.4 billion transport package.

Several Wellington city councillors have said they felt they were presented with a "take it or leave it" deal ahead of the council, saying Mayor Justin Lester told them Genter had threatened to resign over the project.

Lester has denied this, while Genter has said she never threatened to resign.

National has for weeks demanded the release of the letter amid speculation that Genter's influence had seen a second Mt Victoria Tunnel put on the back burner in the Let's Get Wellington Moving package.

Meanwhile, a second Terrace tunnel and the trenching of Karo Drive didn't make the final cut at all.

Asked if the contents of the letter were explosive on Newstalk ZB this morning, Ardern said: "No, I don't believe so. I'm not particularly bothered about it either, I have to say."

She said she had not seen the letter but was aware of its content, saying that National was playing politics.

"I think it's in some peoples' interest to make a bigger deal out of that than it is."

Genter has refused to release the letter to protect free and frank discussions between ministers, and Ardern backed this up, saying its release would set a dangerous precedent.

"We have to be able to, as ministers, have discussions and policy discussions amongst ourselves before decisions are made, and if all of that's exposed all of the time, it makes it harder for us to do our jobs."

She defended the Government's record of being open and transparent, saying it regularly released ministerial diaries and Cabinet papers.

"But we still need to make sure we can run the Government ... making sure we can still get free and frank advice and have some frank discussions."

The Chief Ombudsman is investigating whether the letter should be released, and Genter has said she would abide by the ombudsman's decision.